This invention relates to alarm systems and, more particularly, to intrusion alarm systems that are useful for conveniently and inexpensively providing protection against unauthorized entry into given area.
A commonly used intrusion alarm system for detecting unauthorized entry into buildings is the electrically wired type wherein all doors and windows are wired together in one or more common circuits such that when the electrical circuit is broken, as could occur with an unauthorized entry, an alarm or signal device is activated. Such systems can be quite sophisticated, often incorporating fail-safe or anti-defeat circuitry whereby a high degree of reliability is provided. However, since skilled electricians are required to install and service these systems and since local building codes often impose expensive restrictions on wiring buildings, the installation and maintenance of such wire systems can be quite costly.
To reduce the comparatively high costs of such wire systems, various types of unwired systems using radiant beam communication instead of wiring have been employed, including the use of battery-powered radio transmitters at each of the doors and windows. The last-mentioned type radiant beam system, however, can also be comparatively expensive since a separate battery-powered radio wave or sonic transmitter is usually required for each window and door. Furthermore, such systems can be relatively unreliable due to battery failure and, thus, require frequent inspection, testing and servicing. A further disadvantage of these radiant wave systems is that the alarms may often be inadvertently triggered by spurious noises or spurious radio signals since the more highly selective the system, the greater is its cost and complexity.
Another type of so-called unwired radiant beam system is described in U.S. Pat. Nos. 3,714,647 Litman and 3,805,257 Litman et al, wherein signal devices (transmitters) are described which incorporate multilamp photoflash units. The preferred units are the percussive type devices sold under the name "MAGICUBE", which are produced by the assignee of the present invention. Triggering the percussive flashlamps in these units is accomplished via a spring-loaded pivotal arm which moves in response to some external activation, e.g., pulling of an attached cord or chain. In addition to providing a highly intense flash, e.g., 2,000 beam candle power seconds, the devices are optically coupled to an electric circuit which includes a photovoltaic cell or similar light detector which becomes activated upon receipt of the intense flash of light from the fired photoflash lamps. Assuming a plurality of light pulse transmitters are employed to protect a given area, a centralized light pulse detector is spaced from and in optical communication with all of the transmitters. In order to avoid a false alarm response to ambient lighting conditions, the detector includes a discriminator circuit which responds only to the predetermined transient characteristic of the transmitted light pulse from the photoflash units. Upon discriminatingly detecting the transmitted light pulse, the detector circuitry activates any one or more of a variety of alarm signals. The detector circuits may be energized by either AC or DC sources, or both.
A common disadvantage of above-discussed optically coupled alarm systems employing electronic light-activated components spaced from light-source transmitters involves the possibility of physical interruption of the activating light path by a window shade, drape, or item of furniture, etc. Such an interruption, of course, prevents the transmitted light signal from activating the necessary alarm warning. Further, the detector circuitry can still be comparatively complex and costly, even in the case of the system described by the Litman patents.